<!DOCTYPE html>
<html lang="zh_CN">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script type="text/javascript">
        window.onload = function(){
            var a = document.getElementById("123");
            a.onchange = function(){
                alert("你选择的男神已经改变！");
            }
        }
    </script>
</head>
<body>
    你选择的男神是：
    <select id="123">
        <option >刘光辉</option>
        <option >吴彦祖</option>
        <option >陈冠希</option>
    </select>    
</body>
</html>